Holding a farewell party for a kaafir is a kind of honouring and respecting him, and they do not deserve to be honoured because they have disbelieved in Allaah and offended Him.
Imam Ahmad narrated with a saheeh isnaad from Abu Moosa that he said: I said to ‘Umar: “I have a Christian scribe.” He said, ‘What is wrong with you, may Allaah doom you?! Have you not heard that Allaah says (interpretation of the meaning): ‘O you who believe! Take not the Jews and the Christians as Awliyaa’ (friends, protectors, helpers), they are but Awliyaa’ of each other’ [al-Maa’idah 5:51]? Why don’t you employ a haneef (i.e., a Muslim)?” I said: “O Ameer al-Mumineen, I benefit from his writing and his religion is his own affair.” He said: “I will not honour them when Allaah has humiliated them, and I would not want to be close to them when Allaah has cast them away.”
See Majmoo’ al-Fataawa, 25/327.
‘Umar ibn al-Khattaab also said with regard to the Christians in particular: “Humiliate them but do not wrong them, because they have offended Allaah in such a manner that no other people have ever done.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said, concerning the ruling on holding a farewell party for a kaafir when his work ends, and the ruling on offering condolences to a kaafir, and the ruling on attending the festivals of the kuffaar:
This is a question which includes several issues:
1 – Holding farewell parties for these kaafirs is undoubtedly a kind of honouring them and showing regret that they are leaving. All of that is haraam for the Muslim. The Prophet (peace and blessings of Allaah be upon him) said: “Do not initiate the greeting of salaam with the Jews and Christians, and if you meet them in the road push them towards the narrowest part of it.” The person who truly believes cannot honour any of the enemies of Allaah, and the kuffaar are the enemies of Allaah according to the text of the Qur’aan. Allaah says (interpretation of the meaning):
“Whoever is an enemy to Allaah, His Angels, His Messengers, Jibreel (Gabriel) and Mikaa’eel (Michael), then verily, Allaah is an enemy to the disbelievers”
2 – Offering condolences to a kaafir if one of his loved ones – a friend or relative – dies. There is a difference of opinion among the scholars concerning this issue. Some of the scholars say that it is haraam to offer condolences to them, and some say that it is permissible. Some of them added further details and said that if that serves an interest, such as the hope that they may become Muslim and it may ward off their evil which could not be done except by offering condolences, then it is permissible, otherwise it is haraam.
The more correct view is if that offering condolences to them is regarded as a kind of honouring them, then it is haraam, otherwise we should look at what is in the best interests.
3 – Attending their festivals and taking part in their celebrations. If these are religious festivals such as Christmas then it is haraam to attend them, beyond any shadow of a doubt. Ibn al-Qayyim (may Allaah have mercy on him) said: It is not permissible to attend them according to the consensus of the scholars, and this was clearly stated by the fuqaha’ who followed the four imams in their books. And Allaah is the Source of strength.
Majmoo’ Fataawa Ibn ‘Uthaymeen, 2/303.